NAME

SYNOPSIS

DESCRIPTION

DECLARE allows a user to create cursors, which can be used to retrieve a small number of
rows at a time out of a larger query. After the cursor is created, rows are fetched from
it using FETCH(7).
Note
This page describes usage of cursors at the SQL command level. If you are trying to
use cursors inside a PL/pgSQL function, the rules are different — see Section 40.7,
“Cursors”, in the documentation.

PARAMETERS

name
The name of the cursor to be created.
BINARY
Causes the cursor to return data in binary rather than in text format.
INSENSITIVE
Indicates that data retrieved from the cursor should be unaffected by updates to the
table(s) underlying the cursor that occur after the cursor is created. In PostgreSQL,
this is the default behavior; so this key word has no effect and is only accepted for
compatibility with the SQL standard.
SCROLL
NO SCROLL
SCROLL specifies that the cursor can be used to retrieve rows in a nonsequential
fashion (e.g., backward). Depending upon the complexity of the query's execution plan,
specifying SCROLL might impose a performance penalty on the query's execution time.
NO SCROLL specifies that the cursor cannot be used to retrieve rows in a nonsequential
fashion. The default is to allow scrolling in some cases; this is not the same as
specifying SCROLL. See NOTES for details.
WITH HOLD
WITHOUT HOLD
WITH HOLD specifies that the cursor can continue to be used after the transaction that
created it successfully commits. WITHOUT HOLD specifies that the cursor cannot be
used outside of the transaction that created it. If neither WITHOUT HOLD nor WITH HOLD
is specified, WITHOUT HOLD is the default.
query
A SELECT(7) or VALUES(7) command which will provide the rows to be returned by the
cursor.
The key words BINARY, INSENSITIVE, and SCROLL can appear in any order.

NOTES

Normal cursors return data in text format, the same as a SELECT would produce. The BINARY
option specifies that the cursor should return data in binary format. This reduces
conversion effort for both the server and client, at the cost of more programmer effort to
deal with platform-dependent binary data formats. As an example, if a query returns a
value of one from an integer column, you would get a string of 1 with a default cursor,
whereas with a binary cursor you would get a 4-byte field containing the internal
representation of the value (in big-endian byte order).
Binary cursors should be used carefully. Many applications, including psql, are not
prepared to handle binary cursors and expect data to come back in the text format.
Note
When the client application uses the “extended query” protocol to issue a FETCH
command, the Bind protocol message specifies whether data is to be retrieved in text
or binary format. This choice overrides the way that the cursor is defined. The
concept of a binary cursor as such is thus obsolete when using extended query protocol
— any cursor can be treated as either text or binary.
Unless WITH HOLD is specified, the cursor created by this command can only be used within
the current transaction. Thus, DECLARE without WITH HOLD is useless outside a transaction
block: the cursor would survive only to the completion of the statement. Therefore
PostgreSQL reports an error if such a command is used outside a transaction block. Use
BEGIN(7) and COMMIT(7) (or ROLLBACK(7)) to define a transaction block.
If WITH HOLD is specified and the transaction that created the cursor successfully
commits, the cursor can continue to be accessed by subsequent transactions in the same
session. (But if the creating transaction is aborted, the cursor is removed.) A cursor
created with WITH HOLD is closed when an explicit CLOSE command is issued on it, or the
session ends. In the current implementation, the rows represented by a held cursor are
copied into a temporary file or memory area so that they remain available for subsequent
transactions.
WITH HOLD may not be specified when the query includes FOR UPDATE or FOR SHARE.
The SCROLL option should be specified when defining a cursor that will be used to fetch
backwards. This is required by the SQL standard. However, for compatibility with earlier
versions, PostgreSQL will allow backward fetches without SCROLL, if the cursor's query
plan is simple enough that no extra overhead is needed to support it. However, application
developers are advised not to rely on using backward fetches from a cursor that has not
been created with SCROLL. If NO SCROLL is specified, then backward fetches are disallowed
in any case.
Backward fetches are also disallowed when the query includes FOR UPDATE or FOR SHARE;
therefore SCROLL may not be specified in this case.
Caution
Scrollable and WITH HOLD cursors may give unexpected results if they invoke any
volatile functions (see Section 35.6, “Function Volatility Categories”, in the
documentation). When a previously fetched row is re-fetched, the functions might be
re-executed, perhaps leading to results different from the first time. One workaround
for such cases is to declare the cursor WITH HOLD and commit the transaction before
reading any rows from it. This will force the entire output of the cursor to be
materialized in temporary storage, so that volatile functions are executed exactly
once for each row.
If the cursor's query includes FOR UPDATE or FOR SHARE, then returned rows are locked at
the time they are first fetched, in the same way as for a regular SELECT(7) command with
these options. In addition, the returned rows will be the most up-to-date versions;
therefore these options provide the equivalent of what the SQL standard calls a “sensitive
cursor”. (Specifying INSENSITIVE together with FOR UPDATE or FOR SHARE is an error.)
Caution
It is generally recommended to use FOR UPDATE if the cursor is intended to be used
with UPDATE...WHERECURRENTOF or DELETE...WHERECURRENTOF. Using FOR UPDATE
prevents other sessions from changing the rows between the time they are fetched and
the time they are updated. Without FOR UPDATE, a subsequent WHERE CURRENT OF command
will have no effect if the row was changed since the cursor was created.
Another reason to use FOR UPDATE is that without it, a subsequent WHERE CURRENT OF
might fail if the cursor query does not meet the SQL standard's rules for being
“simply updatable” (in particular, the cursor must reference just one table and not
use grouping or ORDER BY). Cursors that are not simply updatable might work, or might
not, depending on plan choice details; so in the worst case, an application might work
in testing and then fail in production.
The main reason not to use FOR UPDATE with WHERE CURRENT OF is if you need the cursor
to be scrollable, or to be insensitive to the subsequent updates (that is, continue to
show the old data). If this is a requirement, pay close heed to the caveats shown
above.
The SQL standard only makes provisions for cursors in embedded SQL. The PostgreSQL server
does not implement an OPEN statement for cursors; a cursor is considered to be open when
it is declared. However, ECPG, the embedded SQL preprocessor for PostgreSQL, supports the
standard SQL cursor conventions, including those involving DECLARE and OPEN statements.
You can see all available cursors by querying the pg_cursors system view.

EXAMPLES

To declare a cursor:
DECLARE liahona CURSOR FOR SELECT * FROM films;
See FETCH(7) for more examples of cursor usage.

COMPATIBILITY

The SQL standard says that it is implementation-dependent whether cursors are sensitive to
concurrent updates of the underlying data by default. In PostgreSQL, cursors are
insensitive by default, and can be made sensitive by specifying FOR UPDATE. Other products
may work differently.
The SQL standard allows cursors only in embedded SQL and in modules. PostgreSQL permits
cursors to be used interactively.
Binary cursors are a PostgreSQL extension.